This invention relates to ultrasound diagnostic scanning where ultrasonic energy illuminates internal organs of the human body in real time and echoes received from the soft organ tissues are transduced into electrical signals and then processed to form two-dimensional cross-sectional images that are displayed upon a TV monitor or like display device.
Ultrasound systems known as phased arrays have been used for some time for medical imaging and have been described, for example, in U.S. Pat. Nos. 4,140,022 and 4,550,607. Two basic scan and display formats have generally been used in combination with planar linear transducer arrays in which the face of individual transducer elements are positioned in a plane parallel to each other and generally have uniform element spacing.
Two-dimensional images have been formed with planar linear arrays by linear scanning where ultrasonic beams on parallel acoustic lines normal to or at an angle to the face of a transducer array are propagated by single transducer elements or by selected groups of transducer elements shifted across the array. Linear scanning, with parallel lines, has the field of view determined by the physical aperture of the transducer. Its advantage is a large field of view (FOV) near the transducer face. However, a large field of view requires a large physical aperture of active transducer elements which may create problems of access and good skin contact.
The other scan and display format which is typically used for planar linear transducer arrays is a sector. In a sector format, the elements are spaced much closer together, typically at half-wavelength or so intervals. This permits the acoustic scan lines to be steered at angles to the planar array without generating grating lobes and allows both the size of the transducer array to be decreased and the field of view to be increased at deeper depths compared to linear formats.
An analogous method of scanning has been used for non-planar or curved linear arrays where the transducer elements are positioned along an arc with radius of curvature R and the scan format consists of a set of scan lines, each normal to the curved pa face of the transducer array. That format is a consequence of unsteered scan lines which are generated in a way analogous to linearly scanning a planar linear array. The physical characteristics of curved linear arrays define a field of view which is determined by acoustic lines substantially normal to the face of the array. The field of view can be increased by using an array with smaller radius of curvature; however resolution is impaired and thereby degraded as compared to a less curved array. Also, a large field of view in the near-field requires a large aperture of active transducer elements. A large field of view in the far-field requires some combination of a large aperture and/or a smaller radius of curvature with the attendant loss of resolution.
Geometry considerations which apply to planar transducer arrays also apply to curved linear arrays. In applications where little or no "steering" at angles not normal to the transducer face is required, the element-to-element spacing can be large, greater than a wavelength or so. In applications where significant steering is required or desired, the element spacing must be less, on the order of half a wavelength or so. Active aperture width considerations depend upon grating lobe and element rolloff behavior.
All the foregoing display formats, as well as mechanical and waterpath scanning, have the field of view defined completely by the physical characteristics of the array. In none of these prior art scanning formats is the field of view substantially altered by situation-dependent software control.
Earlier applications Ser. Nos. 478,573 and 552,019 describe improvements in scanning and display formats for phased arrays and the disclosures in those applications are incorporated herein by reference. Those improvements are applicable to curved linear arrays and permit steering of scan lines in a particular way which can be used advantageously to increase or reduce the field of view, or can cause an entire family of scan lines to be shifted or rotated in a consistent and useful manner.